This invention is directed toward method and apparatus improvements for temperature-conditioning workpieces such as elongated thermoplastic parts which are to be subsequently further shaped into molecularly oriented hollow articles such as containers.
In U.S. Pat. No. 3,754,851, a system is disclosed for blowing articles from molded preforms which are brought to orientation temperature in an intermediate conditioning step. In this approach heat is removed from the preform during conditioning and such has become known in the art as a "cool-down" process. It is likewise known to add heat to preforms to bring them up to orientation temperature prior to finish forming as is typically disclosed in U.S. Pat. No. 3,715,109 and other related prior art, and such has become known in the art as a "reheat" process. In high speed, high capacity forming lines utilizing either of these approaches, it is clearly desirable to optimize the temperature-conditioning part of such techniques in order to keep fabricating costs related to this step at an absolute minimum. Thus, it is desirable to process a large number of parts at the same time, to minimize handling and reorienting movement of the distortable parts during conditioning, to provide flexibility of conditioning to accommodate different input temperature and to expose each part as uniformly and completely as possible to the heat sink to minimize exposure time and complexity of apparatus.
Though the above-mentioned prior art has generally been successful in implementing cool-down and reheat processing, it is deficient in one or more aspects of the temperature-conditioning phase, especially when such conditioning is considered in the environment of a high speed, continuous, large capacity forming line.
Also, with respect to cool-down processes such as disclosed in U.S. Pat. No. 3,754,851, it has been noted that thickness variations in the wall of the thermoplastic preform traceable back to a non-homogeneous melt wherein portions during extrusion are more fluid than others, are carried over into the final molding step and appear as similar variations in the finished article. Such variations adversely affect performance especially when the article is a container such as a bottle intended for holding pressure. This differential thickness occurring in the circumferential direction is most pronounced when the preform is shaped by blowing from an extruded parison (though it occurs to a lesser extent in injection molding), and should be accommodated, especially if blown preforms are to represent a viable path to oriented containers for pressurized applications.